I was playing with a Motorola G72 and noticed this feature by accident: double-tap on the back of the phone to trigger a custom action.

Here is a video showing this feature

This is not the same as double tapping the fingerprint scanner. This phone has a front-facing under-display scanner.

It's very neat and handy, I hope other phones implement this too.

Update: See this comment for a link to an app that enables the feature on many phones.